In the Greek language the term can apply to men or women; but in modern English it is in use only for men, while nun is used for female monastics.

Although the term monachos (“monk”) is of Christian origin, in the English language it tends to be used analogously or loosely also for both male and female ascetics from other religious or philosophical backgrounds.

The term monk is generic and in some religious or philosophical traditions it therefore may be considered interchangeable with other terms such as ascetic. However, being generic, it is not interchangeable with terms that denote particular kinds of monk, such as cenobite, hermit, anchorite, hesychast, solitary.

The first famous Christian known to adopt the life in a desert was St. Anthony the Great (251-356). Anthony lived alone as an anchorite in the Egyptian desert until he attracted a circle of followers, after which he retired further into the desert to escape the adulation of men. He is said[by whom?] to have been the first to go out into the desert for the sole purpose of pursuing God in solitude.

As the idea of devoting one's entire life to God grew, more and more monks joined him, even in the far desert. Under St. Anthony's system, they each lived in isolation. Later, loose-knit communities began to be formed, coming together only on Sundays and major feast days for Holy Communion. The concept of monks all living together under one roof and under the rule of a single person — that is, monasticism as such — is attributed to St Pachomius (c. 292-348). At this same time, St. Pachomius' sister became the first woman to lead a monastery of women, or convent. Christian monasticism spread throughout the Eastern Roman Empire. At its height it was not uncommon for monasteries to house upwards of 30,000 monks.

As Christianity grew and diversified, so did the style of monasticism. In the East, monastic norms came to be regular. Monasticism came to be accepted in the West as well. In the beginning, Western monasticism followed much the same pattern as its Eastern forebears, but over time the traditions diversified.

In the Eastern Orthodox, Oriental Orthodox and Eastern Catholic Churches, monasticism is a far more common lifestyle than in the Roman Catholic Church and holds a very special and important place. The Orthodox Church measures its health by the quality of its monks and nuns. Orthodox monastics separate themselves from the world in order to pray unceasingly for the world. They do not, in general, have as their primary purpose the running of social services, as is common in Western Christianity, but instead are concerned with attaining theosis, or union with God. However, care for the poor and needy has always been an obligation of monasticism, so Orthodox monasteries are not normally "cloistered" like some contemplative Western houses are, though the level of contact will vary from community to community. Orthodox hermits, on the other hand, have little or no contact with the outside world.

Orthodox monasticism does not have religious orders as are found in the West, nor do they have Rules in the same sense as the Rule of St. Benedict. Rather, Eastern monastics study and draw inspiration from the writings of the Desert Fathers as well as other Church Fathers; probably the most influential of which are the Greater Asketikon and Lesser Asketikon of St. Basil the Great and the Philokalia, which was compiled by St. Nikodemos of the Holy Mountain and St. Makarios of Corinth. Hesychasm is of primary importance in the ascetical theology of the Orthodox Church.

Most communities are self-supporting, and the monastic’s daily life is usually divided into three parts: (a) communal worship in the catholicon (the monastery's main church); (b) hard manual labour; and (c) private prayer, spiritual study, and rest when necessary. Meals are usually taken in common in a sizable dining hall known as a trapeza (refectory), at elongated refectory tables. Food is usually simple and is eaten in silence while one of the brethren reads aloud from the spiritual writings of the Holy Fathers. The monastic lifestyle takes a great deal of serious commitment; those newcomers with romantic notions about this sort of lifestyle usually do not last more than a few days. Within the cenobitic community, all monks conform to a common way of living based on the traditions of that particular monastery. In struggling to attain this conformity, the monastic comes to realize his own shortcomings and is guided by his spiritual father in how to deal honestly with them. Attaining this level of self-discipline is perhaps the most difficult and painful accomplishment any human being can make; but the end goal, to become like an angel on earth (an "earthly angel and a heavenly man", as the church hymns put it), is the reason monastics are held in such high esteem. For this same reason, Bishops are almost always chosen from the ranks of monks.

Eastern monasticism is found in three distinct forms: anchoritic (a solitary living in isolation), cenobitic (a community living and worshiping together under the direct rule of an abbot or abbess), and the "middle way" between the two, known as the skete (a community of individuals living separately but in close proximity to one another, who come together only on Sundays and feast days, working and praying the rest of the time in solitude, but under the direction of an elder). One normally enters a cenobitic community first, and only after testing and spiritual growth would one go on to the skete or, for the most advanced, become a solitary anchorite. However, one is not necessarily expected to join a skete or become a solitary; most monastics remain in the cenobuim the whole of their lives.